Conventional shot peening or blasting is often used to treat concrete or metal by blowing or mechanically impelling particles of steel or iron against a surface. On metal surfaces, the individual particles produce shallow, rounded overlapping dimples in the surface. Conventional shot peening, however, requires extensive blasting equipment. For applications requiting mobility, this blasting equipment is not particularly mobile and the particles are not easily collected for recirculation.
The use of rotating flaps with peening particles attached thereto, known as rotary peening, has proven to be effective for stress relief, surface conditioning, and removal of coatings from surfaces. The process eliminates the use of free shot to peen the surface. Additionally, no solvents are required to loosen surface coatings prior to the rotary peening operation.
A rotary peening apparatus 10 (shown in FIG. 1 ) includes a cylindrical hub 12, including hub ends 14, 16, and opposing mounting flanges 18. The hub 12 is adapted for mounting on a shaft or arbor (not shown), for rotation therewith about a central axis A--A. The hub 12 includes a plurality of guides 22 that are spaced about the perimeter of the hub 12 to provide flap slots 24, which extend parallel to the axis A--A and are adapted to receive retaining ends 17 of peening flaps 30, such as are illustrated in FIG. 2. Disposed within the retaining ends 26 are a plurality of keeper pins that assist in retaining the peening flaps 30 within the flap slot 24. Several flaps are typically arranged in a side by side relationship within each flap slot 24.
One or more peening particle supports 34 is attached to a distal end 32 of a peening flap 30 by shank 35, as illustrated in FIG. 2. Each peening particle support 34 includes a plurality of peening particles 42 protruding therefrom, which particles impact the surface being treated when the apparatus is rotated. The peening particle support 34 simulate the individual particles of conventional shot peening. Such peening flaps are available from Minnesota Mining and Manufacturing Company, of St. Paul, Minn., and are known commercially as Heavy Duty Rolo Peen flaps, Type B, Type C, and Type D. The general construction of the peening flap is further described in U.S. Pat. No. 5,203,189 and the construction of the peening particle support bases is further described in U.S. Pat. No. 5,179,852.
One potential disadvantage of a rotary peening process arises if the peening particle support 34 are circumferentially aligned with each other. If the peening particle support 34 are spaced evenly across the face of the peening flap, an area on the surface or workpiece that is between adjacent peening particle support 34 may not be contacted. Consequently, such an arrangement can result in a plurality of ridges or troughs on the surface or workpiece which is a phenomenon typically called tracking. While a grooved surface finish may be desirable for some applications, such as slip resistance and water drainage, it is unacceptable for other applications.
It has been found that increasing the number of peening particles on a particular peening particle support 34 produces a smoother surface with less tracking. The tradeoff, however, is that increasing the number of peening particles on the peening particle support 34 tends to produce a less aggressive abrasive and increases the required dwell time to achieve the same level of surface treatment. Examples of prior art arrangements of peening particles are shown in FIGS. 3A-3D. As is clear from FIGS. 3A-3D, the peening particles are arranged symmetrically across the surface of the peening particle support 34.
Alternately, a spacer ring 50 having circumferentially spaced pin members 52 that are adapted to cooperate with the flap slots 24 and with the peening flaps 30 may be attached to the hub 12 illustrated in FIG. 1. The circumferentially arranged pins 52 are designed to offset the peening flaps 30 with respect to one another to minimize tracking. The spacer ring 50 is further discussed in U.S. Pat. No. 5,284,039. While the above configuration has significantly improved the surface finish created using a rotary peening process, further improvements are desirable.